Nanostructures

ABSTRACT

A fast method of creating nanostructures comprising the steps of forming one or more electrically-charged regions ( 5 ) of predetermined shape on a surface ( 1 ) of a first material, by contacting the regions with a stamp for transferring electric charge, and providing electrically charged nanoparticles ( 7 ) of a second material, and permitting the particles to flow in the vicinity of the regions, to be deposited on the regions.

[0001] This invention relates to a method of forming structures of smalldimensions, for example of nanometer dimensions—commonly known asnanostructures—and also relates to methods involving interaction ofsmall particles, especially nanometer dimensioned particles withmaterial surfaces.

[0002] Hitherto, small-scale photonic or electronic devices have beenfabricated using photolithographic processing techniques. As sizes arereduced, it becomes difficult to form the individual geometric featuresof these devices at a sufficient degree of resolution due to the need toemploy radiation of ever-shorter wavelengths to expose the photoresist.

[0003] A process that presses a mould into a thin thermoplastic polymerfilm on a substrate to create vias and trenches with a minimum size of25 nm is disclosed in “Imprint of sub-25 nm vias and trenches inpolymers” Chu et al, Applied Physics Letters 67(21), Nov. 20, 1995,pages 3114-3116.

[0004] Nanometer-sized metal and semiconductor particles (nanoparticles)may be regarded as potential components for photonic or quantumelectronic devices. Fabrication of these devices requires not onlydeposition but also positioning of nanoparticles on a substrate. Thereare many different ways of creating nanometre-scale structures usingparticles or clusters as building blocks, such as deposition from asuspension using capillary forces, which gives two- andthree-dimensional arrays of crystal-like structures of particles.

[0005] Nanometer-scale chains of metal clusters have been fabricatedwith a resolution better than 200 nm. They nucleate at the boundary ofthe substrate and lines of photoresist during deposition ofcopper—“Microfabrication of nanoscale cluster chains on a patterned Sisurface”, Liu et al, Applied Physics Letters, Oct. 5, 1995, p 2030-2032.

[0006] “An arrangement of micrometer-sized powder particles by electronbeam drawing”, Fudouzi et al, Advanced Powder Technol., 1997, vol. 8,no. 3, pp251-262, reports that electrically charged lines on the scale20 μm may be written in an insulating surface. It is shown that chargedsilica spheres (5 μm diameter) in a suspension can be controllablydirected towards such charged lines.

[0007] On the topic of electrically charging surfaces, “Electrostaticwriting and imaging using a force microscope” Saurenbach, IEEETransactions on Industry Applications, Volume 28 No. 1, January 1992,page 256 discloses the use of an electrostatic force microscope having atungsten microscope tip, arranged to touch a polycarbonate surface witha small voltage to transfer charge to the surface in order to produce“charge spots” of micrometer dimensions.

[0008] “Charge storage on thin SrTrO₃ film by contact electrification”Uchiahashi et al, Japanese Journal of Applied Physics, Volume 33 (1994),pages 5573-5576 discloses charge storage on thin film by contactelectrification, by using an atomic force microscope. It was possible todiscriminate between charge dots spaced about 60 nm apart. The processis intended for non-volatile semiconductor memories.

SUMMARY OF THE INVENTION

[0009] It is an object of the present invention to provide an improvedmethod by which devices having very small geometric features may befabricated.

[0010] The concept of the present invention is to induce an electriccharge in very small, as small as nanometric-dimensioned areas, on asurface, preferably by contacting a metallic tool in a controlled manneron an insulating substrate. As a second step in the invention,nanometric-dimensioned particles in an aerosol or in liquid phase arethen influenced by the regions of electric charge on the substrate inorder to be deposited on the substrate or otherwise to interact with thesubstrate as explained below.

[0011] In a first aspect, the invention provides a method comprising thesteps of forming one or more electrically-charged regions ofpredetermined shape on a surface of a first material, by contacting saidregions with a tool means for transferring electric charge, andproviding particles of a second material, and permitting the particlesto flow in the vicinity of said regions, to interact in a predeterminedmanner with the electric charge of the said regions.

[0012] In a second aspect, the invention provides apparatus for carryingout a method comprising tool means for contacting one or more regions ofpredetermined shape on a surface of a first material in order totransfer electric charge thereto, and means for permitting particles ofa second material to flow in the vicinity of said regions, to interactin a predetermined manner with said regions.

[0013] In a further aspect, the invention provides a method, comprisingthe steps of forming one or more electrically-charged regions ofpredetermined shape on a surface of a material, providing particles ofnanometric dimensions, and permitting the particles to flow in thevicinity of said regions to interact in a predetermined manner with saidregions.

[0014] For the purposes of the present specification, “particles ofnanometric dimensions” is intended to means particles having a diameterof 300 nanometers or less. As preferred for most applications, theparticle diameter is 50 nanometers or less, and as further preferred, insome applications, for example optoelectronics, the particle diameter is10 nanometers or less.

[0015] The tool means may be a press or stamp having a contoured surfaceof dimensions as large as millimeters or as small as nanometers, whichis arranged to contact the surface of the substrate, and has aconfiguration conforming to the desired pattern or configuration ofelectric charge to be deposited on the substrate. The press or stamp maybe of a rigid material, or a resilient material, e.g. a metal coatedrubber material.

[0016] A significant advantage of employing a stamp is that a complexconfiguration of electrically charged regions of predetermined shape,extending over a wide area, may be formed in a single operation. Theprocess of the invention is therefore very much faster to carry out thanother methods, such as electron beam drawing or writing.

[0017] Alternatively the tool may take the form of a needle, rod orother elongate object which is drawn across the surface in a desiredpath to create the desired pattern of electric charge. The tool may bethe tip of a scanning probe microscope. The tool will usually be ofmetal but can be of any other suitable rigid material having a workfunction which is such in relation to the work function of the firstmaterial to permit charge flow to the surface of the first material. Thefirst material is commonly an insulating material, but may besemiconducting or of any material which is such as to hold the appliedelectric charge for a sufficient length of time to permit the method ofthe invention to take place.

[0018] In addition to the locally charged regions, deposition of thesecond material may be assisted by application of an electrostaticprecipitation field.

[0019] Preferably, the particles of the second material have a secondelectrical charge of opposite sign to the first. Alternatively, theparticles of the second material may be of the same sign as that of thefirst electric charge, and the pattern of the deposited second materialis determined by the repulsion from the one or more electrically chargedregions.

[0020] The requirement that the particles be charged may in some casesbe relaxed—particles may become polarised in an electric field and willbe attracted towards electrostatically charged objects due to anelectric field gradient.

[0021] In another application, electrically neutral nanometric particlesmay be projected against a surface, each to absorb one or more chargecarriers, and to rebound from the substrate in an electrically chargedcondition.

[0022] As well as contact charging, other mechanisms may be employed forthe creation of locally-charged regions, including inducing a chargepattern by irradiation with photons, e.g. by synchrotron light using amask, or inducing a charge pattern by laser interference on a polarsemiconductor surface.

[0023] The particles of a second material may be formed by any suitableprocess. A preferred process of producing the particles in aerosol formis described below. Alternatively other processes such as laser ablationmay be employed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Preferred embodiments of the invention will be now be describedmerely by way of example with reference to the accompanying drawings, inwhich:

[0025]FIG. 1 is a schematic diagram illustrating the method of theinvention;

[0026]FIGS. 2a to 2 c is a sequence illustrating the application ofelectrical charge to receptor regions of an insulating surface inaccordance with the invention;

[0027]FIG. 3 is a schematic view of a deposition chamber (precipitator)for an aerosol nanoparticle generator, for the method of the invention;

[0028]FIG. 4 illustrates, in diagrammatic form, an aerosol nanoparticlegenerator, described in our co-pending PCT Application No. GB98/03429;

[0029]FIGS. 5a-c to 9 are scanning electron micrographs of surfaces ofmaterials having particulate deposits thereon, formed in accordance withthe invention; and

[0030] FIGS. 10-20 are schematic drawings showing various embodiments ofthe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] Referring now to the FIG. 1 of the drawings, the surface of asilicon wafer 3 is oxidised to produce a silicon dioxide layer 1, andlocalised regions 5 of negative charge are imprinted on the surface.Nanoparticles 7 formed in an aerosol unit are impressed with a positivecharge and are attracted to the locally charged regions 5 of the silicasurface layer, with the assistance of a local electric field F.

[0032] One method of applying the local charge to the surface isillustrated in FIGS. 2a to 2 c. A nanoprinting stamp 9 is made from aconducting material (or from an insulator coated with a metal), and isbrought into contact with the insulating surface 1. The stamp 9 hasprotrusions 11 formed on its contact surface in a predeterminedconfiguration. The width of these protrusions may range from dimensionsof nanometers up to the macroscopic millimetre range, preferablyfabricated by electron beam lithography. The height of the protrusionsis not material to the region definition. After contact, localisedcharged regions are left on the surface 1 of the substrate exactlymirroring the dimensions and structures of the stamp protrusions.

[0033] The basis of this method is that charges cross the interface ofan insulator and a metal brought into contact. After the metal isremoved, a charge is retained on the insulator. The sign and amount ofcharge transferred depends approximately linearly on the work functionor Free Energy of the metal in relation to the work function or FreeEnergy of the insulating substrate. The amount of charge may beincreased by providing a potential difference between the metal and theinsulator. It is estimated that, with the method of this preferredembodiment, 10⁵ charges per square micrometer or less are transferred.

[0034] The substrate, which now has a pattern of charge on its surface1, is placed in a deposition chamber or precipitator for an aerosolnanoparticle generator as shown in FIG. 3. The generator producesparticles with a controlled charge, either positive or negative. If theaerosol particles have a polarity opposite to that of the charge on thesurface of the substrate, the particles will preferentially depositwhere the substrate is charged (FIG. 1), whereas particles with the samepolarity as the surface charge pattern will be repelled from thepattern, and are deposited in the spaces between the locally chargedregions. In the case of no applied electric field, particles withopposite charge states will still deposit where the substrate ischarged, whereas particles with the same polarity as the surface chargepattern will not be deposited.

[0035] Further processing steps may then be undertaken to fix theparticles permanently to the surface.

[0036] Referring to FIG. 3 there is shown a chamber 20 which is electricgrounded having an inlet 22 in its upper wall for receiving particles 7in the form of an aerosol. A electrode 24 in the chamber is connected toa source of potential 26 in order to generate an electric field betweenthe electrode and the walls of the chamber. The electrode 24 is mountedon an insulating tube 28. The electrically charged sample 1 is placed onthe upper surface of the electrode 24. The sample is a distance of somecentimetres from the opening 22.

[0037] In use, particles 23 entering the chamber through opening 22 flowtowards sample 1. The electric charge on the sample as shown in FIG. 1may be sufficient to attract the particles for deposition. However, asshown, the deposition may be assisted by the electric field existingbetween electrode 24 and the walls of chamber 20.

[0038] In this case, the particle deposition may take place (a) byattracting particles of a different polarity to the charged regions and(b) by deflecting particles from the charged regions. In the first caseparticles are deposited at the charged regions but are also deposited onareas in between the regions with lower density and randomly. This isdependent on the distances between the fields, the strength of themacroscopic electric field applied, the particle size, and the particlespeed in the gas flow. In the second case particles would only bedeposited in between the charged regions.

[0039] Furthermore, this embodiment may be adapted so as to use insteadof particles in an aerosol, colloidal particles from the liquid phase,which will also be attracted by the charge patterns.

[0040] Other methods of bringing the particles close to the surface,which do not rely on an macroscopic electric field, may be used, e.g.,inertial impaction or thermophoresis.

[0041] Whilst the creation of charge patterns can easily be demonstratedfor insulating surfaces, the method may also be used for semiconductorand metal surfaces, although the amount of charges and the time durationof charges might be smaller as compared with insulating surfaces.

[0042] An additional feature is that the substrate itself or the activesurface layer can be very thin—just a few nanometres, for example 50 nm.This facilitates the creation of a charge pattern on one side while theparticles are deposited on the other side of a substrate. This mayenable the stamping apparatus shown in FIG. 2 to be incorporated in thedeposition chamber shown in FIG. 3 in that the sample may be held inplace within the chamber and the stamping apparatus is brought againstthe underside of the sample to impart a pattern of electric charge (theupper surface of the electrode may form the stamp). This charge willthen be effective to attract aerosol particles streaming down onto theupper surface of the sample. The thickness of this substrate is limitedsimply by the dielectric constant of substrate material, the number ofcharges stored in the surface and the electric mobility of theparticles, which itself is a function of the particle size, the numberof charges on the particles and the medium in which the particle issuspended. Thus, thin foils could be used as substrate materials.

[0043] In order to generate electrically charged particles fordepositing on the substrate surface in the apparatus as shown in FIG. 3,the apparatus of FIG. 4 is employed in this embodiment. This is anaerosol generator capable of producing an aerosol with a volume flow of1680 cm³/min and a particle concentration of around 5×10⁵ cm⁻³ was usedfor particle generation. In FIG. 4, a furnace F1 generates metallicparticles by sublimation. An electrical charger C1 is placed after thefurnace to charge the aerosol particles. Size selection takes place in adifferential mobility analyser DMA1. DMA apparatus exploits the factthat the electrical mobility of singly charged particles is amonotonically increasing function of particle size. While sending a flowof electrically charged particles in a perpendicular electric field, thefield causes particles to be attracted to one capacitor plate. Particleswith higher electrical mobility will be precipitated on the nearestportion of the plate and those with lower mobility will be carried alongwith the main flush flow. Only those with the correct mobility, andhence particles size, will be attracted to the facility of the samplingslit where they are swept out by the gas stream flowing through theslit. The DMA can produce particles with a closely controlled dimension,to within a standard deviation of a few percent. These particles areconducted to further furnace F2 where they are mixed with a hydride gasin order to produce further particles of a further composition. Theseparticles are subject to a close dimensional control in a further DMA2.For a given particle diameter, a distribution of diameters of ±0.2 ofthe diameter is achieved. The diameter of the particles may be as smallas 5 nm, or even molecular size. These particles are conducted to adeposition chamber DC, which is as shown in FIG. 3. An electrometer E1and a pump Pu are connected to measure the particle concentration and tocreate a gas flow therein for flowing the particles into the depositionchamber or precipitator.

[0044] The carrier gas is ultra pure nitrogen at ambient pressure androom temperature. Due to the generation process, the particles carryeither one positive or one negative charge. For deposition, the aerosolflows into the apparatus shown in FIG. 3. A stagnation point flow wasestablished over the substrate. An electric field guides the chargedparticles towards the substrate surface where they are deposited. In theabsence of this field, the particles follow the streamlines of thecarrier gas and no deposition occurs. Thermal wet oxidised silicon (111)with oxide thickness of 500 nm and a plane surface were used assubstrates. The silicon was p+ doped with 0.01 to 0.02 Ω-cm resistivity.No special cleaning process was carried out except that of removingcoarse particles by blowing with nitrogen.

[0045] As an alternative to using a stamp, contact charging of thesubstrate surfaces may be carried out with a stainless steel needle thatis slid over the substrate surface without applying pressure, bothneedle and substrate being earthed

[0046] For sample evaluation, scanning electron microscopy (SEM) wasused to obtain the particle arrangement on the substrate surfaces.

[0047]FIG. 5 shows the homogeneous distribution of particles that isobtained after deposition of negatively charged, 30 nm indium particlesin the homogeneous electric field of the electrostatic precipitator with150 kV/m electric field strength. The particles are attracted to thesurface by the macroscopic electric field. Their macroscopicdistribution is homogeneous over the whole sample area. The samebehaviour is obtained for the deposition of positively charged particlesin a negative field.

[0048] When negatively charged particles are deposited at 150 kV/m on asubstrate, which has previously been patterned with lines of negativesurface charges, as shown in FIG. 6, particle-free zones show up withinthe homogenous particle distribution. FIG. 6 is a scanning electronmicrograph that shows that these zones are approximately 10 μm in width.At their border is a narrow transition region (shown in the inset) witha width of around 300 nm in which the particle density increases fromzero to the mean density found on the rest of the substrate. The patternof particle free zones observed after the deposition corresponded to thepattern applied with a steel needle. This indicates a negative chargingof the contact area between substrate and needle.

[0049] In the case of deposition of positively charged particles with ahomogeneous electric field of ˜150 kV/m on a negative charged substrate,particles are deposited as shown in FIG. 7 in an approximately 10 μmline 80 lying in the centre of a 200 μm particle free zone 82. Theparticle density within this line 80 is higher by a factor ofapproximately 5-10 compared to the mean density on the rest of thesample. Again, the borders of the different areas are very sharp. Thelines were situated where the steel needle traversed the substrate. Inregions 84, remote from line 80, deposition by electrostaticprecipitation occurred.

[0050] For the fabrication of microelectronic components, it is oftendesirable to cover certain areas of a substrate selectively with asingle material, such as gold, while the rest has to remain clean. Thismeans that it is preferable to avoid the uncontrolled coverage of thesubstrate caused by the electric field of the electrostaticprecipitator. Depositing positively charged particles on a chargedsubstrate with the electrostatic precipitator turned off, i.e. noelectric field applied, the surprising result is that the amount ofcharges on the substrate is sufficient to attract the particles from thegas flow. This means that the deposition becomes very selective and onlythe parts of the sample that are charged will be covered with particles.Line width of approximately 10 μm can be achieved. When the same processwas carried out with negatively charged particles no particles at allwere deposited.

[0051] When handling the substrate under ambient conditions the surfacewill have a contamination layer consisting mainly of water. During thecontact electrification, charges are trapped in the silicon oxidesurface as well as in the contamination layer. The latter are mobile andcan move within the contamination layer. This leads to a broadening ofthe charge patterns on the surface. Using surfaces without thiscontamination layer improves the sharpness of the boundary betweencharged and non-charged regions. As preferred therefore, measures andmeans are employed to remove or prevent the formation of the watercontamination zone, such as heating the substrate in a water-freeatmosphere.

[0052] In one specific embodiment, silicon with a 1 μm layer of oxidewas pressed against a Compact Disc (CD) master. A CD master is a metalplate with protrusions corresponding to where the depressions in the CDwill be. These protrusions are on the scale of 1 μm. The result afteraerosol particle deposition is shown in scanning electron micrographs(FIGS. 8a to c). This demonstrates that particles gather on the contactelectrified spots. FIG. 8a is the case for a uniform deposition with nolocally charged regions. FIG. 8b, on the same scale as FIG. 8a, showsthe deposition on locally charged regions by pressing with the CDmaster. FIG. 8c is a reduced scale view of FIG. 8b.

[0053] In accordance with a specific embodiment of the invention, asshown in FIG. 9, lines were made by gently sliding a metal needleagainst an insulating (SiO₂) surface. This shows that it is possible tocollect positively or negatively charged particles with or withoutapplying an external electric field in the precipitator. A resolutionbelow 50 nm may be attained.

[0054] It is possible to cover a surface with nanometre resolution withsubstances that can be electrically charged and dispersed in a carriergas. The size range of the building blocks ranges from several hundredsof nanometres down to individual molecules. The flexibility of thisprocess permits the creation of structures with resolutions from themillimetre size range (e.g. sensors) down to the 100 nm or even lowersize range (e.g. quantum devices). This makes the connection between themacroscopic and the nanoscale world possible in one process step.Another result that could be observed is that it is possible to arrangeparticle chains of different particle densities closely beside oneanother.

[0055] For the fabrication of electronic nanostructures, it is desirablethat the charging process should neither destroy nor contaminate thesubstrate surface. Provided that one chooses the correct materialcombination, e.g., a sufficiently hard material is pressed against asofter surface, then the surface will elastically deform withoutpermanent deformation, provided the contact pressure is sufficientlylow. A hard material will not damage the substrate since the harmlesscontact is just sufficient to create the charge pattern and no forceswill be applied. Actually, creating surface defects, e.g. scratches,will ruin the effect of contact charging. With a softer material, i.e.where the bonds between the surface and the bulk atoms are not strong,it is possible that material might remain on the surface after thecontact.

[0056] The limitations for structural resolution of the method forfabricating distinct structures on a surface are mainly given by thenumber of charges stored in the surface, the number of particlesdeposited, and the electrical mobility of the particles. The electricalmobility is a function of the particle size, the number of chargescarried by the particle and the medium the particle is suspended in.

[0057] This invention finds particular application in circumventing thelimits of conventional photolithography. As circuits get ever smaller,the number of layers of metal lines (called vias) used to connect thedevices on the chip increases, becoming one of the largest components ofthe cost of chip manufacture. Each layer of metal requires a separatelithographic step, where photoresist is applied, exposed, and developed,followed by evaporation of metal, and finally lift-off of excess metal.Here, it permits the fabrication of leads with nanometre dimensionwithout any lithography step and without destroying the underlyingstructure.

[0058] Even the subsequent deposition of different material or differentmaterial sizes is possible by first creating a charge pattern anddeposition of one sort of particles followed by a second charge patterncreation and another particle deposition. Here, a fixation step for theinitially deposited particles might be necessary, such as annealing.

[0059] The present invention may also be utilised to replace the veryfine lithography employed in making chemical or biological sensors. Itmay also be used for fabricating catalytic structures.

[0060] Optical detectors with sub-picosecond response times have beenmade with (very slow) electron beam lithography and metallisation. Inthis way, interdigitated Metal-Semiconductor-Metal junctions are formedwith lateral metal-metal spacing of below 50 nm. With method of thisinvention, an entire optoelectronic device may be fabricated veryefficiently, such as optoelectronic components based on nanoparticles.For some of these, ordering of the particles on the scale of thewavelength of the light is crucial. Among such components are quantumdot based laser and light emitting diodes.

[0061] The method may also be used in photonic bandgapmaterials—particles placed in arrays ordered on the scale of thewavelength of light which exhibit a band gap for the photons, so thatsome wavelengths are not permitted to pass. This has applications inoptical communication.

[0062] The invention also finds application in the fabrication ofinterference colouring and anti-reflective coatings and for theconstruction of nanostructured surface, which exhibit uniquetribological properties, such as wear resistance.

[0063] Further application could be found in the fabrication of magneticstorage devices, flash memory devices, electroluminescence displays.Also for the controlled seeding of the growth of nanotubes andnanowhiskers, the present invention can be applied.

[0064] Additionally, projecting neutral particles with a higher speedtowards surface regions charged by the method would lead to a chargetransfer from the surface to the particles permitting particlesscattered by the surface to acquire a charge.

[0065] The invention also finds application in the removal of particlesfrom a gas or a liquid.

[0066] Referring now to FIG. 10, this shows examples of Coulomb—blockadedevices created by the method in accordance with the invention. FIG. 10ashows chains of nanoparticles 100 forming wireless single-electron logicbased on electrons hopping between nanoparticles. FIG. 10b shows singleelectron transistor structures with central nanoparticles 100, 102influenced by electrodes 104, 106.

[0067]FIG. 11 is a schematic diagram of the method in accordance withthe invention applied to fabricating nanometer-size metallic circuitstructures. A stamp 110 (shown conceptually) having a predeterminedshape 112 is pressed against a substrate 114 to create a correspondingpattern of charged regions 116. Metallic particles 118 of an oppositecharge type are then deposited on the substrate to adhere to the pattern116. After an annealing step, the particles merge to form continuousmetallic features 119.

[0068] Referring now to FIG. 12 this shows creation of multi-metallicsurface structures by controlled nanoparticle deposition in accordancewith the invention. A stamp 120 is pressed against a substrate 122 toform a pattern of charged regions 124 on the substrate, on whichoppositely charged nanoparticles 125 are deposited. The particles arethen fixed on to the substrate by a suitable process. A further stamp126 with a different stamp pattern is pressed against substrate 122 toproduce a second pattern 128 of charged regions. This permitsnanoparticles 129 of a different type to land on the second chargedregion.

[0069] Referring to FIG. 13, showing the fabrication of quantum-dotlasers by the method in accordance with the invention, wherein stamp 130with metallised protrusions 132, having dimensions of less then 20nanometers, is pressed against a substrate 134. The material of thesubstrate is in this example the n-type part of a laser structure;alternatively the substrate may constitute the p-type part of the laser.The metallised protrusions create charged regions or spots on thesubstrate to permit particles 136 to be deposited on the localisedcharge regions to create a pattern of n-type laser-active quantum dots138. After epitaxial overgrowth of the particles as at 139 with a p-typesubstrate to create the laser structure, the system is ready for finalprocessing.

[0070] The procedures of FIGS. 12 and 13 may be essentially combined, inthat repeated operations of depositing p- or n-type particles, formingparts of laser structures, can be carried out, each operation employingparticles of a different diameter, and hence laser characteristics, e.g.wavelength. Finally, an epitaxial overgrowth is carried out, as in FIG.13.

[0071] Referring now to FIG. 14, there is shown the fabrication ofphotonic band gap materials by a method in accordance with the inventionwherein a stamp 140 having metallised protrusions 142 with lateraldimensions of the order of one quarter of the wavelength of the light inquestion (for example about 10 micrometers) is pressed against asubstrate 144 to create charged regions of a similar pattern. Micrometersized particles 146 are then deposited on the substrate to accumulate onthe locally charged regions. By prolonged deposition, particles willland on top of each other to create filaments 148 or chains ofparticles. This creates filaments in a desired lattice structure havingdimensions of the order of the wavelength of light, whereby to create,by Bragg reflection, photonic band gaps for transmitted light.

[0072] Referring to FIG. 15, there is shown the fabrication of nanotubearrays by the method in accordance with the invention, wherein a stamp150 with protrusions 152 having lateral dimensions less than 20nanometers is pressed against a substrate 154 to create locally chargedregions. Nanometer sized particles 156 of the opposite charge typedeposited on the substrate to adhere to the locally charged regions.Using the nanoparticles as seeds, arrays or filaments of carbonnanotubes 158 can be grown by chemical vapour deposition methods. Thishas application in field emission applications.

[0073] Referring to FIG. 16, there is shown the fabrication of nanorodarrays in accordance with the invention, wherein a stamp 160 withprotrusions 162 having lateral dimensions of less than 20 nanometers ispressed against a substrate 164 to create locally charged regions.Nanometer sized particles 166 are deposited on the locally-chargedregions, and these particles are used as seeds to create filaments ornanorods, for example semiconducting or magnetic materials, which aregrown by chemical vapour deposition methods.

[0074] Referring to FIG. 17, there is shown a method of electricallycharging of aerosol particles. A stamp 170 having metallised protrusionswith lateral dimensions of the order of centimetres 172 is pressedagainst a substrate 174 to create a charged region 176. Neutral aerosolparticles are directed to the surface at high speed, thus rebounding onthe substrate and taking away respective charged units from the chargedregion 176. Alternatively the substrate may not be electrically charged.The aerosol particles are nevertheless effective to “extract” electricalcharge from the substrate by the impaction process.

[0075] Referring to FIG. 18, there is shown a method for removing sootparticles from exhaust gas streams wherein a cylinder 180 of insulatingmaterial, positioned in an exhaust pipe, of for example an engine, has arotating metallic brush 182 mounted centrally on pipe 180 by supports184. The metal brush has metal filaments 186 contacting the inner wall,and as it rotates it charges the inner surface of the wall with negativecharge. The cylinder may be of silicon oxide, glass or ceramic. At anearlier stage, particles in the exhaust gas are positively charged,either as the result of the combustion process or by a separate meanssuch as a charger. These charged particles are then deposited on thecylinder wall. The brush functions to wipe the particles from the wallinto a exhaust channel 188 for further treatment.

[0076] Referring how to FIG. 19, there is shown a method of seedingepitaxial self-assembled dots for two and three dimensional arrays ofquantum dots, by the method in accordance with the invention. As shownin FIG. 19a locally charged regions 190 are created by the method inaccordance with the invention. Using an epitaxial method, self-assembleddots 192 are formed on the locally charged dots. The epitaxial methodmay be molecular beam, a chemical beam or metal-organic vapour phasedepitaxy, or any combination thereof. An insulating layer 194 is thengrown over particles 192, and this process is repeated to create threedimensional arrays 196 of quantum dots.

[0077] In FIG. 19b the method is somewhat similar to that shown in FIG.19a and similar parts denoted by the same reference numeral. However, inan initial step nanoparticles, for example tungsten, 191 are depositedon the electrically charged arrays by the method in accordance with theinvention. Using an epitaxial method, a thin buffer layer 194 is grownto cover the particles. In the next epitaxial step, self-assembled dots192 are formed on top of the embedded particles as described in the restof FIG. 19a. A prolonged epitaxial process will create three dimensionalarrays 196 of quantum dots.

[0078] Referring to FIG. 20, a flash memory structure comprises asubstrate 200 with source and drain electrodes 202. A gate structure 204overlies a conducting channel 206. The gate structure comprises an oxidelayer 208, a nanoparticle layer comprising nanoparticles 210 in anepitaxial overgrowth 212, and a further oxide layer 214, with a finalmetallic gate electrode 216. The nanoparticles 210 have the capacity forcharge storage, and may be of any suitable material. In the formation ofthe structure, the oxide layer 208 is initially grown over the surfaceof the substrate, and then the nanoparticles are applied by the stampprocess, as above. Successive further steps of epitaxial overgrowth andselective etching create the structure shown.

1. A method comprising the steps of forming one or moreelectrically-charged regions (5) of predetermined shape on a surface (1)of a first material, by contacting said regions with a tool means (9)for transferring electric charge, and providing particles (7) of asecond material, and permitting the particles to flow in the vicinity ofsaid regions, to interact in a predetermined manner with said regions.2. A method according to claim 1, wherein said particles are ofnanometric dimensions.
 3. A method according to any preceding claim,wherein the second material is different from the first material.
 4. Amethod according to any preceding claim, wherein said one or moreelectrically-charged regions are charged with a charge of a first sign,and the particles of the second material are charged with charge of asecond sign, the second sign being opposite to that of the first sign.5. A method according to any of claims 1 to 3, wherein said one or moreelectrically charged regions are charged with a charge of a first sign,and the particles of the second material are charged with a charge of asecond sign, the second sign being the same as that of the first.
 6. Amethod according to claims 4 or 5, wherein said particles of a secondmaterial each carry one or more electric charges.
 7. A method accordingto any preceding claim, wherein an electric field is provided in adirection towards said surface so as to enhance the flow of saidparticles towards said first surface.
 8. A method according to claim 7,wherein the electric field induces a charge polarisation of saidparticles, which is effective to deposit the particles on the surface.9. A method according to any preceding claim, wherein said particles aredeposited on the surface in areas determined by said one or moreelectrically-charged regions in order to fabricate a structure.
 10. Amethod according to claim 9, wherein the particles are deposited on saidone or more electrically-charged regions.
 11. A method according toclaim 9, wherein the particles are deposited on said surface on areasother than said one or more electrically-charged regions.
 12. A methodaccording to any of claims 1 to 3, wherein said particles of a secondmaterial are electrically neutral, and are projected against saidregions (176) to absorb electrical charge units, and to rebound fromsaid surface in an electrically-charged condition (178).
 13. A methodaccording to any preceding claim, wherein the tool means comprises astamp having a contoured surface, for contacting the surface of thefirst material, with a configuration conforming to the shape of saidregions.
 14. A method according to any of claims 1 or 12, wherein thetool means comprises an elongate object such as a needle or rod which ispressed against the surface of the material and drawn across the surfacein a desired path to define said regions.
 15. A method according toclaim 14, wherein the tool means is the tip of a scanning probemicroscope.
 16. A method according to any preceding claim, wherein saidsurface is prepared so as to have no significant water or otherconductive contamination.
 17. A method of fabricating a materialconfiguration according to any preceding claims, wherein said particlesof a second material are arranged to flow as an aerosol.
 18. A method offabricating a material configuration according to any of claims 1 to 16wherein said particles of a second material are arranged to flow as asuspension in a liquid.
 19. A method according to any preceding claim,wherein the second material is metallic, and the particles are depositedon the first material and subsequently annealed to the surface.
 20. Amethod according to any preceding claim, wherein in a second stage ofthe method, the steps set forth in claim 1 are repeated with a differentone or more electrically-charged regions (128) of a differentpredetermined shape or size.
 21. A method according to any of claims 1to 18, wherein the first material forms a first part (134) of a laserstructure, the particles of the second material are deposited on thefirst material and form a second part of the laser structure (138), andincluding a further step of epitaxial overgrowth (139) of the particlesof the second material to form the laser structure.
 22. A methodaccording to claims 20 and 21, wherein particles of the second materialare deposited in successive steps, each step comprising particles of adifferent size and laser characteristics.
 23. A method according to anyof claims 1 to 18, including depositing said particles of a secondmaterial on the surface of the first material for a prolonged period tocreate filaments (148, 158) of the second material upstanding from thesurface.
 24. A method as claimed in any preceding claim, includingproviding a voltage bias between said tool means and said surface.
 25. Amethod according to any of claims 1 to 18, modified by (a) forming onsaid one or more electrically charged regions, self assembled dots (192)by an epitaxial method, (b) growing an intermediate layer on theself-assembled dots (194), and repeating said steps (a) and (b) adesired number of times (196).
 26. A method according to any of claims 1to 18, wherein a flash memory structure is created by depositingparticles on an insulating layer covering a substrate, embedding theparticles in an insulating layer, and then selectively etching thelayer, and forming electrodes for a flash memory structure.
 27. A flashmemory structure, formed by the method of claim
 26. 28. A nanometrescale electronic or optoelectronic device formed by depositing at leastone material on one or more electrically-charged regions of a surface,as claimed in any of claims 1 to
 25. 29. A surface having modifiedtribological properties formed by depositing material on one or moreelectrically-charged regions of a surface, in accordance with any ofclaims 1 to
 25. 30. A surface having modified optical properties formedby depositing material on one or more electrically-charged regions of asurface, in accordance with any of claims 1 to
 25. 31. A method,comprising the steps of forming one or more electrically-charged regions(5) in a surface (1) of a first material of a predetermined shape, saidregions being formed by irradiating the surface with photons, providingparticles (7) of a second material, and permitting the particles to flowin the vicinity of said one or more electrically-charged regions to bedeposited on the surface in a pattern determined by said regions.
 32. Amethod, comprising the steps of forming one or more electrically-chargedregions (5) of a predetermined shape in a surface of a polarsemiconductor material, being formed by laser interference on saidsurface, providing particles (7) of a second material, and permittingthe particles to flow in the vicinity of said regions to be deposited onthe surface in a pattern determined by said regions.
 33. A method,comprising the steps of forming one or more electrically-charged regions(5) of predetermined shape on a surface (1) of a material, providingparticles (7) of nanometric dimensions, and permitting the particles toflow in the vicinity of said regions to interact in a predeterminedmanner with said regions
 34. A method according to claims 31-33, andfurther including the steps according to any of claims 2 to
 25. 35. Amethod comprising providing particles of nanometric dimensions (178),and permitting the particles to impact against and rebound from asurface (174) of a substrate so that the particles absorb an electricalcharge.
 36. A method according to claim 35 wherein the surface of thesubstrate is electrically charged (176).
 37. A method of removingparticles from a stream of gas, comprising providing a surface (180)having electrical charges by contacting the surface with a tool means(182, 186), allowing the stream of gas to flow over the surface so thatthe particles are deposited on the electrically charged surface, andthen removing the particles from the surface.
 38. A method according toclaim 37, wherein the tool means is a rotating brush, which is alsoeffective in a subsequent rotation to remove the particles from thesurface and to deposit them in a collection area (188).
 39. Apparatusfor carrying out a method comprising tool means (9) for contacting oneor more regions (5) of predetermined shape on a surface (1) of a firstmaterial in order to transfer electric charge thereto, and means (20,22) for permitting particles (7) of a second material to flow in thevicinity of said regions, to interact in a predetermined manner withsaid regions.
 40. Apparatus according to claim 39, including aerosolmeans (F1-DMA2) for producing particles of nanometric dimensions. 41.Apparatus according to claim 40, wherein said aerosol means is arrangedto electrically charge said particles.
 42. Apparatus according to any ofclaims 39-41, including means (26) for producing an electric field in adirection towards said surface so as to enhance the flow of saidparticles towards said surface.
 43. Apparatus according to any of claims39-42, wherein the tool means comprises a stamp having a contouredsurface, for contacting the surface of the first material, with aconfiguration conforming to the shape of said regions.
 44. Apparatus forcarrying out a method comprising tool means (9) for forming one or moreelectrically charged regions (5) of predetermined shape on a surface (1)of a first material, and means (20, 22) for permitting particles (7) ofa second material to flow in the vicinity of said regions, to interactin a predetermined manner with said regions.
 45. Apparatus for removingparticles from a stream of gas, comprising a surface (180), a tool means(182, 186) for contacting the surface for transferring electrical chargethereto, means for allowing the stream of gas to flow over the surfaceso that the particles are deposited on the electrically charged surface,and means (182, 186) for removing the particles from the surface. 46.Apparatus according to claim 45, wherein the tool means is a rotatingbrush, which is also effective in a subsequent rotation to remove theparticles from the surface and to deposit them in a collection area(188).